Filters and the like that use a bed of filter media to remove solids from liquids are well known. Such systems typically include a liquid distribution system that collects liquid after it is filtered in one flow direction and distributes clean liquid in a reverse flow direction through the filter media to effect a cleaning process known as backwashing. The backwashing process may simultaneously or alternately introduce air or another gas through the media to enhance the cleansing effect, known in the art as "air scouring." For example, U.S. Pat. No. 593,666--Jewell discloses a filter system that uses central manifold and a series of laterally extending branch pipes to distribute backwash air through strainers. The air manifold and laterals are separate and the air manifold is disposed within the central manifold and laterals that carry liquid, except that the strainers are not directly connected to the orifices in the air laterals. U.S. Pat. No. 668,344--Wilson, discloses an underdrain structure that combines separate channels to carry backwash liquid and backwash air, the air and water laterals are arranged in alternating fashion in a single plane along the bottom of the filter bed. The water main, however, enters at the bottom of the bed while the air main enters at the top of the bed and is split into two sections that extend downwardly to the bottom of the filter bed. The section of the air main disposed at the bottom of the filter contains the air laterals. U.S. Pat. No. 1,988,279--Hungerford discloses an air grid that rests on supports extending from the bottom of a filter structure such that the air grid is disposed above the underdrain/water grid piping and is located at the interface between a gravel layer and a layer of filter media. the Hungerford patent explains that if the air grid is disposed below the gravel, the gravel layer is disturbed during air scouring and the filter will fail in subsequent operation. On the other hand, if the gravel layer is omitted, the fine filter media will clog the orifices in the underdrain. The relatively smaller orifices in the air grid are not clogged if they are of a diameter smaller than the diameter of the fine media particles.
In addition to grids comprised of pipe, fluid transport systems for underdrain filters are known that comprise a series of underdrain blocks that connect to create internal fluid conduits. Layers of gravel and/or filter media are laid atop the underdrain blocks to complete the filter system. U.S. Pat. No. 3,468,422--Camp discloses such an underdrain and uses interconnected ceramic blocks that create an internal conduit for distributing both liquid and air, the latter being supplied into each block by individual pipes connected to an air header disposed beneath the bottom of the blocks. U.S. Pat. No. 3,817,378--Ross discloses a filter system wherein air is introduced into underdrain blocks using a separate air grid comprised of an air header and air laterals disposed beneath the blocks, and also uses a sperate air grid disposed atop a layer of fine filter media to agitate effluent.
Additionally, U.S. Pat. No. 5,019,259--Hambley discloses a filter underdrain formed from sheet metal that provides conduits for both backwash liquid and backwash air. The Hambley patent explains that structures and methods relating to underdrain and backwash systems are useful not only in filter systems, but also in other types of systems such as upflow and downflow contact clarifiers and filters, activated carbon contactors, ion exchange units, iron removal units, catalyst bed contactors and neutralizing media contactors. Thus, it is known in the art that the term "underdrain" is to be interpreted broadly, and as used herein, is to be interpreted to encompass all such systems and similar systems wherein it is useful to provide an air grid for distributing a flow of air or other gas across a surface. For example, U.S. Pat. No. 5,167,840--Jaccarino discloses a clarifier that combines upflow and downflow filter media. An air grid is located at the bottom of the upflow filter bed, but is similar in construction to the backwash air grids disclosed in the prior art systems discussed above, that is, the air grid comprises an air conduit and a series of perforate air laterals mounted to a structure beneath liquid carrying conduits disposed beneath the filter media.
Certain prior art filters, clarifiers and similar systems did not include an air grid for air scouring or similar operations. It would be desirable to include such a feature, since backwash operations and ultimately the performance of the filter or other system would be enhanced. It is therefore an object of the present invention to provide methods and apparatus whereby and air grid can be retrofitted into an existing system. Additionally, whether for a retrofit or a new system, previous designs suffered from drawbacks associated with integrating the backwash air conduits within or integral with the liquid conduits. In many systems structures that are not level or where equipment has been inaccurately specified or constructed, among other things, it is often difficult, if not impossible, to achieve efficient operation. The need to flow liquid in two directions and a also flow gas in one of the directions, with even distribution of all fluids in all directions and through essentially the same structure severely limits the ability of the construction team to modify the liquid and gas distribution systems. Thus, it would be desirable to provide a system wherein the liquid and gas flow through different conduits in different structures so that lengths and diameters can be altered and/or baffles, reducers, restrictions or other modifications can be readily made to one system without structural alteration of the other. It is therefore an additional object of the present invention to provide a modular air grid system that may be readily used with a wide variety of filters and other fluid handling systems. Thus, a further objective of the present invention is that an air grid, whether new or a retrofit, can be inserted into a system, activated and evaluated, and changed if necessary. Achieving this goal would permit the "trial and error" solutions often necessary in actual applications without undue expense or burden. Additionally, certain existing air headers use a nipple to connect a "tee" fitting to an air header; air laterals are connected to the remaining legs of the "tee." It would be desirable to simplify this connection and it is thus another object of the present invention to provide a structurally sound and simple connection that permits connection between a header and air laterals.